In order to track open rates there is a cue that must be embedded in the email to allow tracking. Unfortunately, that cue can easily trigger filters and has a considerable impact on deliverability. Because of that, we do not use them. We feel that since you can easily measure the effectiveness of our marketing (i.e. increased showings, attendance at events like open houses and broker opens, increased hits on virtual tours and websites), our customers prefer the higher throughput received by not employing this method.